1. Field of the Invention
This invention relates to an optical semiconductor device module, and more particularly to an optical semiconductor device module applicable to two-way optical communication through wavelength division multiplexing.
2. Description of the Prior Art
FIG. 6 of the accompanying drawings is a schematic view showing a conventional optical semiconductor device module published in Research Report OQE91-108 of The Institute of Electronics, Information and Communication Engineers.
Referring to FIG. 6, this optical semiconductor optical device module comprises a module casing 1 (called "casing 1"), a light emitting element 2, sub-mounts 3, 5, 7, a monitoring photodiode 4, a light receiving element 6, a first lens 8, a lens holder 9, an optical fiber 10, an optical filter 11, a second lens 12, a band-pass filter 13, a sealing glass window 14, and a fiber holder 15. A plurality of terminals 16 are arranged outside the casing 1.
The light emitting element 2 is made of a semiconductor laser chip, and is soldered onto the sub-mount 3. The monitoring photodiode 4 is made of a semiconductor photodiode chip, and is secured on the sub-mount 5. The monitoring photodiode 4 can detect an optical output from the light emitting element 2. The light receiving element 6 is made of a semiconductor photodiode chip, and is soldered onto the sub-mount 7. The sub-mount 7 is soldered to a part of the casing 1 at a position such that the light receiving element 6 can converge beams which arrive via the optical fiber 10 and are reflected by the optical filter 11.
The first lens 8 is attached to the lens holder 9, and converges beams emitted by the light emitting element 2. The converged beams pass through the optical filter 11, and are converged into the optical fiber 10. The lens holder 9 is soldered to the casing 1 at a position such that the beams from the light emitting element 2 are converged into the optical fiber 10. The second lens 12 is soldered to the casing 1 at a position where the beams reflected by the optical filter 11 are converged in the light receiving element 6.
The optical filter 11 includes a glass plate, a multi-layer dielectric film filter, and an anti-reflection film for preventing reflection of the beams from the light emitting element 2. Both the multi-layer dielectric film filter and the anti-reflection film are respectively formed on opposite sides of the glass plate. The optical filter 11 is fixed to the casing 1 by soldering. Beams having particular wavelengths can pass through the bandpass filter 13, which is secured to the second lens 12 using an adhesive.
The sealing glass window 14 is secured to the casing 1 at a position between the optical filter 11 and the optical fiber 10. Fusible glass is used to secure the sealing glass window 14 so as to accomplish air tightness.
The terminals 16 are arranged outside the casing 1 also using the fusible glass so as to accomplish air tightness. In the optical semiconductor device module of FIG. 6, the light emitting element 2, monitoring photodiode 4 and light receiving element 6 are electrically connected to the terminals 16 via a connecting mechanism, not shown.
In FIG. 6, a cover for the casing 1 is omitted. The cover is secured to the casing 1 by low resistance welding. The cover, casing 1 and sealing glass window 14 seal the interior of the casing 1 and maintain it in an air tight condition.
The optical fiber 10 has an obliquely ground tip, and is supported by the fiber holder 15.
Generally, a pair of optical semiconductor device modules are used respectively for a main station and a sub-station in an optical communication system. When the main station uses a wavelength of 1.55 .mu.m for communication to the sub-station, a light emitting element 2 in the module of the main station emits beams whose wavelength .lambda.1 is 1.55 .mu.m. When the sub-station uses a wavelength of 1.3 .mu.m so as to communicate with the main station, a light emitting element 2 of the sub-station emits beams having the wavelength .lambda.1 of 1.3 .mu.m. In the sub-station, beams arriving via an optical fiber 10 have a wavelength .lambda.2 of 1.55 .mu.m.
In the main station, the optical filter 11 is required to transmit the beams whose wavelength .lambda.1 is 1.55 .mu.m and reflect the beams whose wavelength .lambda.2 is 1.3 .mu.m. Further, the bandpass filter 13 has to transmit the beams whose wavelength .lambda.2 is 1.3 .mu.m and reflect the beams whose wavelength .lambda.1 is 1.55 .mu.m. On the other hand, in the sub-station, the optical filter 11 is required to transmit the beams whose wavelength .lambda.1 is 1.3 .mu.m and reflect the beams whose wavelength .lambda.2 is 1.55 .mu.m. Further, the bandpass filter 13 has to transmit the beams whose wavelength .lambda.2 is 1.55 .mu.m and reflect the beams whose wavelength .lambda.1 is 1.3 .mu.m.
The light receiving element 6 may be a ternary photodiode which is sensitive to the beams of wavelength 1.55 .mu.m and 1.3 .mu.m. Therefore, the light receiving elements 6 are the same in both the main station and the sub-station. In both stations, the light emitting elements 2, optical filter 11 and bandpass filters 13 are compatible with the wavelengths allotted in the respective stations they belong to.
In operation, the optical semiconductor device module functions as described below in the main station.
The light emitting element 2 emits beams having the 1.55-.mu.m wavelength, which are converged in the first lens 8. The converged beams then pass through the optical filter 11, and are converged into the optical fiber 10. Thereafter, a light signal is transmitted to the optical semiconductor device module in the sub-station.
In the sub-station, the light signal from the main station is received by the optical semiconductor device module via the optical fiber 10. This light signal has a wavelength of 1.55 .mu.m. The light signal is reflected by optical filter 11 and is converged onto the second lens 12, passes through the bandpass filter 13, and is finally input into the light receiving element 6.
The optical filter 11 has the anti-reflection film on the glass plate, and cannot pass all of the beams but reflects approximately 1% of the beams. Further, the multi-layer dielectric filter film cannot completely separate beams of two separate wavelengths but leaves approximately 0.1% to 1% of the beams as noise components. In other words, weak beams reflected by the optical filter 11 remain as stray beams.
The stray beams are reflected on an inner wall of the casing 1. Then, reflected stray beams pass through the optical filter 11, and are incident onto the second lens 12. Most of these beams are reflected by the bandpass filter 13, and are not incident onto the light receiving element 6. However, some of the remaining stray beams are input to the light receiving element 6, thereby generating noise.
Further, some of the beams emitted by the light emitting element 2 are reflected on an end face of the optical fiber 10, serving as stray beams. The stray beams are also reflected by the anti-reflection film and the multi-layer dielectric filter film of the optical filter 11. The reflected stray beams pass through the second lens 12 and the bandpass filter 13, and are incident onto the light receiving element 6. Most of the stray beams are reflected by the bandpass filter 13 while some of the stray beams pass through the bandpass filter 13 and are received by the light receiving element 2, thereby generating noise.
In a signal transmitting path, there are stray beams caused by an optical connector or the like connected to the optical fiber 10. These stray beams also generate noise similarly to the stray beams present on the end face of the optical fiber 10.
In order to reduce near-end crosstalk, levels of this noise should be 15 dB to 20 dB lower than a level of the light signal detected by the light receiving element 6. For instance, when a light signal detected by the light receiving element 6 has a level of approximately -30 dBm, the noise level should be kept less than -50 dBm overall. Therefore, the near-end crosstalk caused by reflection or the like in the optical semiconductor device module is set to -50 dBm or lower.
The light emitting element 2 emits beams which have a level of approximately 7 dBm. Since most of the beams are converged by the first lens 8, they should be attenuated by 57 dB or more by the optical filter 11, bandpass filter 13 and so on. If the near-end crosstalk, which is caused by reflection in the casing 1, is 57 dB or less, the optical filter 11 is required to have a more excellent performance. This means an increase in the cost of the optical filter 11. Likewise, the bandpass filter 13 is required to have improved performance, which would increase the number of bandpass filters 13 and the manufacturing cost thereof.
Conventional optical semiconductor device modules are prone to problems as described below. To reduce the near-end crosstalk, a highly efficient optical filter 11 is required. Further, it is necessary to use a very efficient bandpass filter 13 or to use a large number of bandpass filters. Thus, the greater the number of bandpass filters or the more efficient bandpass filters are, the more expensive the optical semiconductor device modules become.